The present invention relates generally to a scanning antenna. More particularly, the present invention relates to a plasma controlled scanning antenna operable in the microwave (xcexcW) or millimeter wave (MMW) bands, for example.
Scanning antennas are necessary to form and scan an electromagnetic beam. Historically, there have been generally two types of scanning antennas, either mechanically scanned or electronically scanned. Mechanically scanned antennas perform scanning by forming a fixed beam with the antenna and physically moving the antenna. Electronically scanned antennas have been based on phased arrays which often employ hundreds to thousands of phase shifters to individual elements or groups of elements.
Mechanically scanned antennas are generally slower than desired and require precision hardware which is often expensive. Because mechanically scanned antennas rely on moving parts, reliability is an issue. Electronically scanned phased array antennas offer many advantages, but the large numbers of phase shifters make such systems costly.
Accordingly, alternative scanning methods have been of recent interest. Generally, these alternative methods are motivated by a desire for higher performance at lower cost. For example, a non-mechanical scanning antenna, without phase shifters, has been developed and is based on a type of Fresnel zone plate. The antenna forms and steers a beam of millimeter wave or microwave radiation using a light-modulated photoconducting wafer. See, e.g., U.S. Pat. No. 5,159,486 to Webb, entitled xe2x80x9cInstrumentation apparatus and methods utilizing photoconductors as light-modulated dielectricsxe2x80x9d; U.S. Pat. No. 5,360,973 to Webb, entitled xe2x80x9cMillimeter Wave Beam Deflectorxe2x80x9d; Webb et al., xe2x80x9cLight-Controlled MMW Beam Scannerxe2x80x9d, Proc. 1993 SBMO International Microwave Conference, Vol. II, Sao Paolo, Brazil, IEEE Cat. No. 93TH0555-3, p. 417; and Webb et al., xe2x80x9cMMW Beam Scanner Controlled by Lightxe2x80x9d, Proc. Workshop on Millimeter-Wave Power Generation and Beam Control, Huntsville, Ala., Special Report RD-AS-944, U.S. Army Missile Command, 1993, p. 333, the entire disclosures of which are incorporated herein by reference.
As another alternative, antennas have been developed which use at least two thin semiconductor reflecting plates (e.g., silicon) which are supported (e.g., on glass) and separated by a synthetic foam spacer of dielectric constant near one. There are, however, disadvantages associated with such technique. The use of two or more plates presents complications which require the spacing of the plates to be controlled. A synthetic foam spacer is fragile and easily damaged either mechanically or by temperature. The use of thin plates, especially in the case of silicon of about 50-200 xcexcm in thickness, makes it difficult to achieve the required plasma density under photo-injection because of the effect of surface mediated recombination in the thin plates. See, e.g., U.S. Pat. Nos. 5,084,707, 5,585,812 and 5,736,966, each to Reits.
Recently, antennas have been disclosed which use a single photoconducting plate, e.g. silicon, and a transparent millimeter wave reflector. See, e.g., Webb et al., xe2x80x9cPhotonically Controlled 2-D Scanning Antenna,xe2x80x9d PSAA-8 Proceedings of the Eighth Annual DARPA Symposium on Photonic Systems for Antenna Applications, The Naval Postgraduate School, Monterey, Calif., Jan. 13-15, 1998 (available from DTIC No. AD-B233444); Webb et al., xe2x80x9cExperiments on an Optically Controlled 2-D Scanning Antenna,xe2x80x9d 1998 Antenna Applications Symposium, Allerton Park, Monticello, Ill., Sep. 16-18, 1998, p. 99; Webb et al., xe2x80x9cOptically Controlled Millimeter Wave Antenna,xe2x80x9d Proceedings International Topical Meeting on Microwave Photonics, Melbourne, Australia, Nov. 17-19,1999, p.275; and Webb et al., xe2x80x9cNovel Photonically Controlled Antenna for MMW Communications,xe2x80x9d Proceedings International Topical Meeting on Microwave Photonics MWP 2000, Oxford UK, Sep. 11-13, 2000, p. 97. However, there is no indication of optimum thickness of the photoconducting plate, the nature of the transparent millimeter wave reflector, or the MMW phase relations of the wafer which are desirable for best performance.
In view of the aforementioned shortcomings associated with existing scanning antennas, there remains a strong need in the art for a further improved scanning antenna.
An improved plasma controlled millimeter wave (MMW) or microwave (xcexcW) antenna is provided in accordance with the present invention. A plasma of electrons and holes is photo-injected into a photoconducting wafer. A special distribution of plasma and a MMW/xcexcW reflecting surface behind the wafer allows the antenna to be generated at low light intensities and a 180xc2x0 phase shift (modulo 360xc2x0) to be applied to selected MMWs/xcexcWs. The selected phase change produces superior performance over similar antennas without the phase change.
As is known, Fresnel zone plates (FZP) are of two general types, blocking and phase correcting. The simplest form of FZP works by blocking radiation. Rays going through different parts of an aperture add in-phase or out-of-phase at a detection point. If those rays which add out of phase are blocked, then there is a large gain in received intensity. Generally the phase conditions which produce a large increase in power are present in a given direction and thus the FZP produces a beam of radiation in that direction.
In previous transmissive-type antennas, a technique was used which involved a transient blocking FZP in which a spatially varying density of plasma of charge carriers, electrons and/or holes, was created by optical injection into a semiconductor or photoconductor wafer. The un-illuminated parts of the photoconductor with no plasma allow incident MMW from a feed behind the wafer to be transmitted through the wafer. In the illuminated regions, however, the photo-injected charge carriers alter the index of refraction of the wafer locally. At sufficient light intensity the plasma density was large enough to substantially block MMW in those local lighted regions; at large enough plasma density the plasma caused the transmitted MMW to asymptotically approach zero in magnitude. The wafer, modified by light in this way, is made to diffract incident radiation into a beam and thus comprised a transient FZP. Because the wafer responds rapidly to changes in optical injection, it is possible to change rapidly transient Fresnel diffractive conditions and thus rapidly change the beam direction.
In accordance with an exemplary embodiment of the present invention, a MMW feed is positioned in front of the wafer and an optically transparent MMW reflecting surface (reflector) is positioned in close proximity to the back surface of the wafer. The reflector is designed to be highly reflecting to MMW but transmit visible or infrared light of a wavelength below the band gap of the wafer in order to photo-inject plasma. A controllable light source behind the reflector can be positioned close to the reflector to minimize the need for focusing optics for the light patterns. The wafer thickness is chosen to be nominally an odd integer multiple of the wavelength of the MMW in the wafer material. With this choice of parameters MMW incident on a lighted region of the wafer containing plasma will be phase shifted by nominally 180xc2x0 from MMW incident on a dark region.
These features of the present invention enable two advantageous modes of operation. One mode is an improved blocking FZP antenna, and the second mode is as a phase correcting FZP which uses all the incident MMW radiation. As a blocking FZP antenna, a low plasma density can be chosen which provides for the principle of destructive interference to be used to completely block the undesired out-of-phase MMW. With proper control of phase in the MMW this blockage can be made to be complete, not just asymptotically approaching zero, and at much lower plasma density than in previous designs. The fact that a lower plasma density is suitable for operation allows for much less light intensity and electrical power to be used.
The second mode of operation, the phase correcting FZP, occurs at higher plasma density for the regions containing the out-of-phase rays. In this case when the plasma density created is large enough, the MMW are reflected from the front surface of the wafer. Because the wafer thickness is nominally an odd integer multiple of the wavelength of the MMW in the wafer material, the MMW reflected from the front surface of the wafer are given a 180xc2x0 phase shift with respect to MMW in the dark regions which make a double pass through the wafer. In this way, the out-of-phase rays are given a 180xc2x0 phase shift and thus constructively interfere in the beam. A large increase in beam power and antenna efficiency results.
The present invention is described primarily in the context of an antenna designed to operate in the MMW band. However, it will be appreciated that the antenna may instead operate in other radio frequency (RF) bands such as the microwave (xcexcW) band. For example, an antenna according to the present invention may be designed to operate anywhere in the range of 4 gigahertz (GHz) to 400 GHZ.
According to one particular aspect of the invention, a plasma controlled reflector antenna is provided. The antenna includes a reflector configured to reflect radio frequency (RF) radiation having a frequency equal to that of an operating frequency of the antenna. In addition, the antenna includes a feed for illuminating the reflector with and/or receiving from the reflector RF radiation at the operating frequency to transmit/receive RF radiation. A Fresnel zone plate (FZP) wafer is also included adjacent the reflector and interposed between the reflector and the feed. The FZP wafer has a thickness substantially equal to n*xcexvac/(4*N), where n is an odd integer, xcexvac is the free space wavelength of RF radiation at the operating frequency, and N is the index of refraction of a material of which the wafer is made, in a non-plasma injected state. Furthermore, the antenna includes a controllable light source for projecting a controlled light pattern onto the FZP wafer to inject selectively plasma into regions of the FZP wafer illuminated by the light pattern, thereby creating regions in a plasma injected state and regions in a non-plasma injected state.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.